OBJECTIVE: Mechanical indentation and ultrasound (US) indentation instruments have been introduced for quantitative assessment of cartilage properties in vivo. In this study, we compared capabilities of these instruments to determine properties of healthy and spontaneously degenerated human patellar cartilage in situ and to diagnose the early stages of osteoarthritis (OA). DESIGN: Six anatomical sites were localized from human patellae (N=14). By determining the force by which the tissue resists constant deformation (F(IND)), a mechanical indentation instrument was used to measure the compressive dynamic stiffness of cartilage. Further, the dynamic modulus (E(US)) and the US reflection coefficient of cartilage surface (R(US)) were measured with an US indentation instrument. For reference, Young's modulus and dynamic modulus were determined from cartilage disks using unconfined compression geometry. Proteoglycan and collagen contents of samples were analyzed microscopically. The samples were divided into three categories (healthy, early degeneration, and advanced degeneration) based on the Osteoarthritis Research Society International (OARSI) OA-grading. RESULTS: Parameters R(US), E(US) and F(IND) were significantly associated with the histological, compositional and mechanical properties of cartilage (|r|=0.28-0.72, n=73-75, P<0.05). Particularly, R(US) was able to discern degeneration of the samples with high sensitivity (0.77) and specificity (0.98). All parameters, except R(US,) showed statistically significant site-dependent variation in healthy cartilage. CONCLUSIONS: US reflection measurement shows potential for diagnostics of early OA as no site-matched reference values are needed. In addition, the high linear correlations between indentation and reference measurements suggest that these arthroscopic indentation instruments can be used for quantitative evaluation of cartilage mechanical properties, e.g., after cartilage repair surgery.